Full Idea
The genus can very often be turned into the differentia, ...so that in place of saying that man is a 'reasonable animal' we could, if language permitted, say that man is an 'animable rational', a rational substance with animal nature.
Gist of Idea
Genus and differentia might be swapped, and 'rational animal' become 'animable rational'
Source
Gottfried Leibniz (New Essays on Human Understanding [1704], 3.03)
Book Reference
Leibniz,Gottfried: 'New Essays on Human Understanding', ed/tr. Remnant/Bennett [CUP 1996], p.292
A Reaction
This is a very telling point which rather undermines any dogmatic approach to what Aristotle says about these sorts of definitions. I don't find this account of definitions very helpful anyway. Leibniz links it to the order of cataloguing.